Calcium-dependence of catecholamine release from bovine adrenal medullary cells after exposure to intense electric fields

Intracellular localization of secretable cAMP in relaying Dictyostelium discoideum cells

Dictyostelium discoideum cells synthesize and secrete the chemoattractant cAMP within minutes after chemotactic stimulation. During development, this signal-relay process is instrumental in cell aggregation, pattern formation, and differentiation. Cyclic AMP is known to accumulate inside the cell before secretion. In this study we investigated the subcellular localization of the nascent cAMP. After chemotactic stimulation at 0 degrees C and subsequent accumulation of intracellular cAMP, the newly synthesized chemoattractant could be released by gently opening cells in two different ways. Both methods make the cytosolic compartment accessible, whereas intracellular compartments surrounded by a membrane remain largely intact. The first method involved rapid lysis by forced passage through a 5-micron pore-size Nuclepore filter. The second technique was electropermeabilization under carefully controlled conditions that ensured the formation of small, stable pores in the plasma membrane. These pores allowed the passage of small molecules, such as cAMP, but not of macromolecules. To confirm the selectivity for the plasma membrane of both methods, we showed that a typical vesicular cell compartment, the lysosome, remained intact. Both procedures immediately released all intracellularly accumulated cAMP. We interpret our results as strong evidence for accumulation of nascent cAMP in the cytosolic compartment rather than in a vesicular compartment before it is secreted. This implies that cAMP secretion takes place via a trans-membrane transport mechanism, rather than by exocytosis.

Stimulation of renin secretion by non-diuretic sulfhydryl reagents

Organomercurial diuretics stimulate renin secretion although the underlying cellular mechanisms remain undefined. Since organomercurials are also known to react with sulfhydryl groups, the present studies determined the effects of sulfhydryl reagents on renin secretion. The effects of the non-diuretic mercurial agent, parachloromercuriphenyl-sulfonate (PCMPS), as well as that of other sulfhydryl reagents, N-ethylmaleimide (NEM), N-phenylmaleimide (NPM) and monobromotrimethylammoniobimane (qBBR), on renin secretion were determined in rabbit renal cortical slices. All four reagents stimulated renin secretion. NEM, which has a high membrane permeability, stimulated secretion to a relatively small extent and its effects were not apparent for at least one hour. Conversely, PCMPS, which is much less permanent than NEM, produced the largest stimulation and these effects were apparent within one hour. The stimulation of secretion by sulfhydryl reagents was independent of the concentration of Ca2+, Na+, and K+ in the incubation media, suggesting that the stimulation is not secondary to alterations of intracellular ion concentrations. These results raise the possibility of direct involvement of sulfhydryl groups of particular membrane protein(s) of the juxtaglomerular (JG) cells in some steps leading to renin secretion, and raise the possibility that sulfhydryl reactivity might in part account for the stimulatory effects of organomercurial and other diuretics.

Ionic and osmotic dependence of secretion from permeabilised acini of the rat pancreas

Many types of secretory granule have been observed to swell as a result of cell stimulation implying a degree of osmotic control, although the regulation of granule fusion with the apical plasma membrane is not clearly understood. In the present study we have investigated the ionic and osmotic dependency of basal and stimulated 3H-protein release from rat pancreatic acini, permeabilised by either digitonin or high voltage electric discharge. Acini were stimulated with either cholecystokinin-pancreozymin octapeptide (CCK-8), carbachol (CCh), or with phorbol ester (TPA) plus cAMP. Stimulated secretion was significantly reduced when 130 mmol/l Cl- in the buffer was replaced by I-, NO3-, SCN- or cyclamate-. Secretion in Cl- buffers was inhibited by the anion transport inhibitor 4,4-diisothiocyanatostilbene-2,2-disulfonic acid (DIDS), by 40% of the control response. Neither Na+ nor N-methyl-D-glucamine+ could replace K+ in the buffer. Ba2+ and quinine, which block K+ conductance pathways, inhibited stimulated secretion by 50%. Finally, stimulated secretion from leaky cells was nearly abolished by doubling buffer osmolarity. The data suggest that when the cell is stimulated, a Cl- and a K+ permeability appear in the zymogen granule membrane and the ions enter down their electrochemical gradients. The increased intragranular osmolarity results in granular swelling which is intimately associated with secretion.

Actin involvement in exocytosis from PC12 cells: studies on the influence of botulinum C2 toxin on stimulated noradrenaline release

Botulinum C2 toxin is known to ADP-ribosylate actin. The toxin effect was studied on [3H]noradrenaline secretion of PC12 cells. [3H]Noradrenaline release was stimulated five- to 15-fold by carbachol (100 microM) or K+ (50 mM) and 10-30-fold by the ionophore A23187 (5 microM). Pretreatment of PC12 cells with botulinum C2 toxin for 4-8 h at 20 degrees C, increased carbachol-, K+-, and A23187-induced, but not basal, [3H]noradrenaline release maximally 1.5-to three-fold, whereas approximately 75% of the cellular actin pool was ADP-ribosylated. Treatment of PC12 cells with botulinum C2 toxin for up to 1 h at 37 degrees C also increased stimulated [3H]noradrenaline secretion, whereas toxin treatment for greater than 1 h decreased the enhanced [3H]noradrenaline release stimulated by carbachol and K+ but not by A23187. Concomitantly with toxin-induced stimulation of secretion, 20-50% of the cellular actin was ADP-ribosylated, whereas greater than 60% of actin was modified when exocytosis was attenuated. The data indicate that ADP-ribosylation of actin by botulinum C2 toxin largely modulates stimulation of [3H]noradrenaline release. Moreover, the biphasic toxin effects suggest that distinct mechanisms are involved in the role of actin in secretion.

Receptor-mediated release of inositol 1,4,5-trisphosphate and inositol 1,4-bisphosphate in rat basophilic leukemia RBL-2H3 cells permeabilized with streptolysin O

Antigen-mediated exocytosis in intact rat basophilic leukemia (RBL-2H3) cells is associated with substantial hydrolysis of membrane inositol phospholipids and an elevation in concentration of cytosol Ca2+ ([ Ca2+i]). Paradoxically, these two responses are largely dependent on external Ca2+. We report here that cells labeled with myo-[3H]inositol and permeabilized with streptolysin O do release [3H]inositol 1,4,5-trisphosphate upon stimulation with antigen or guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) at low (less than 100 nM) concentrations of free Ca2+. The response, however, is amplified by increasing free Ca2+ to 1 microM. The subsequent conversion of the trisphosphate to inositol 1,3,4,5-tetrakisphosphate is enhanced also by the increase in free Ca2+. Although [3H]inositol 1,4,5-trisphosphate accumulates in greater amounts than is the case in intact cells, [3H]inositol 1,4-bisphosphate is still the major product in permeabilized cells even when the further metabolism of [3H]inositol 1,4,5-trisphosphate is suppressed (by 77%) by the addition of excess (1000 microM) unlabeled inositol 1,4,5-trisphosphate and the phosphatase inhibitor 2,3-bisphosphoglycerate. It would appear that either the activity of the membrane 5-phosphomonoesterase allows virtually instantaneous dephosphorylation of the inositol 1,4,5-trisphosphate under all conditions tested or both phosphatidylinositol 4-monophosphate and the 4,5-bisphosphate are substrates for the activated phospholipase C. The latter alternative is supported by the finding that permeabilized cells, which respond much more vigorously to high (supraoptimal) concentrations of antigen than do intact RBL-2H3 cells, produce substantial amounts of [3H]inositol 1,4-bisphosphate before any detectable increase in levels of [3H]inositol 1,4,5-trisphosphate.

G regulatory proteins and muscarinic receptor signal transduction in mucous acini of rat submandibular gland

The involvement of G regulatory proteins in muscarinic receptor signal transduction was examined in electrically permeabilized rat submandibular acinar cells. The guanine nucleotide analog, GTP gamma S, caused the dose dependent hydrolysis of membrane phosphatidylinositol 4,5-bisphosphate to release IP3. This response was insensitive to pertussis toxin treatment and was duplicated by NaF but not by GDP beta S. Enhanced IP3 synthesis was observed with a combination of GTP gamma S and carbachol. Exogenous IP3, as well as carbachol and GTP gamma S, provoked the release of sequestered 45Ca2+ from non-mitochondrial stores. In intact cells, carbachol significantly reduced the level of cyclic AMP induced by the beta-adrenergic agonist, isoproterenol, to 69% of its normal value. Pertussis toxin abolished this inhibitory action of carbachol on cyclic nucleotide levels. These results suggest that muscarinic receptors are coupled to two separate G regulatory proteins in submandibular mucous acini-the pertussis toxin-insensitive Gp of the phosphoinositide transduction pathway associated with elevated cytosolic calcium levels, and the pertussis toxin-sensitive Gi inhibitory protein of the adenylate cyclase complex.

Protein kinase C is not involved in secretion by permeabilized human neutrophils

The generally accepted sequence of intracellular signal transduction involves: (1) cell surface receptor-ligand interactions; (2) activation of G-proteins; (3) activation of phospholipase C, leading to inositol phosphate (IP3), and diacylglycerol production; (4) parallel mobilization of intracellular Ca2+ by IP3, and; (5) activation of protein kinase C (PKC) by diacylglycerol and Ca2+, leading to; (6) cellular responses. Human neutrophils appear to utilize this cascade, at least in general, and some, but not all, elements of the intracellular signal cascade known to be operating in intact cells also function in permeabilized cell systems. We have previously shown that permeabilized neutrophils can be induced to secrete lysosomal enzymes in response to elevated levels of Ca2+ alone and this secretion can be synergistically enhanced by the presence of guanine nucleotides. We now show that Ca2+, in the presence and absence of guanine nucleotides, can stimulate the production of soluble inositol phosphates. Furthermore, neomycin, a putative inhibitor of phospholipase C, can block Ca2(+)-induced secretion. These data thus suggest a role for phospholipase C activity or its products in the transduction process. The next enzymatic activity 'downstream' is PKC. Consequently, we looked at the role Mg-ATP, one of the substrates of PKC, plays in degranulation by permeabilized neutrophils, We found no obligatory role for this nucleotide in the secretory process. We then looked at the activity of oleoyl-acetyl-glycerol (OAG), a synthetic diacylglycerol and PKC agonist, on degranulation. We found that OAG was largely additive with Ca2+. Another PKC agonist, phorbol myristate acetate (PMA), also did not display notable synergy. Finally, inhibitors of PKC activity were not capable of blocking secretion, either in the presence or absence of guanine nucleotides. Thus, while circumstantial evidence seems to point towards a requirement for phospholipase C activation and diacylglycerol production in secretion, we were unable to demonstrate the next putative step in signal transduction, namely activation of PKC.

Transient electropermeabilization of cells in culture. Increase of the cytotoxicity of anticancer drugs

The electropermeabilization (EPN) of living cells allows the uptake of non-permeant molecules and can reveal their potential activity on cells without the constraints of the plasma membrane crossing. We decided to compare the cytotoxicity of some anticancer drugs on electropermeabilized (EP) and non-permeabilized (NEP) cultured DC-3F cells exposed to the drugs for a short time. After EPN, the increase in cytotoxicity varies between 1 and more than 700 times, depending on the usual cell uptake pathway of a given drug. The most relevant increase of toxicity was observed with molecules such as netropsin (200-fold) and bleomycin (700-fold) which in ordinary conditions weakly diffuse through the plasma membrane. Only a 3-5-fold increase of the cytotoxicity was observed with lipophilic drugs able to rapidly diffuse through the plasma membrane (actinomycin D, NMHE) both in the case of drug-sensitive and resistant cell strains. This increased toxicity is clearly related to a facilitated uptake because, after electropermeabilization, the effects of melphalan (a drug which enters intact cells via leucine transporters) are not modulated by the external leucine concentration. Thus, EPN enables us to reveal the intrinsic toxicity of hydrophilic molecules which have a limited access to their intracellular targets. We propose that EPN can be used as a novel screening procedure of new cytotoxic molecules which could be modified thereafter in order to facilitate their cellular uptake.

Agonist-induced changes in cell membrane capacitance and conductance in dialysed pancreatic acinar cells of rats

1. Single acinar cells enzymatically isolated from the rat pancreas were subjected to tight-seal whole-cell recordings. Changes in cell membrane capacitance and conductance were simultaneously recorded using a phase-sensitive detection method. 2. Acetylcholine (ACh, 0.05-0.5 microM) and cholecystokinin octapeptide (CCK-8, 10-50 pM) concomitantly induced transient increases in cell membrane current, capacitance and conductance only when cytosolic Ca2+ was weakly chelated by EGTA (70 microM). These responses were prolonged when the cells were dialysed with a solution containing GTP gamma S (a stable analogue of GTP, 50-100 microM), whereas they were inhibited by dialysing with that containing GDP beta S (a stable analogue of GDP). These results suggest that a type of guanine-nucleotide-binding protein (G-protein) could be involved in ACh- or CCK-receptor signalling. 3. The ACh- or CCK-induced responses (with or without GTP gamma S in the cytosol) were all abolished when a high dose of EGTA (1-2 mM) was injected into the acinar cells. In addition, A23187, a calcium ionophore, induced sustained responses when the cytosolic Ca2+ was weakly buffered by 70 microM-EGTA. These results suggest that the secretagogues regulate the changes in cell membrane capacitance and conductance via an increase and decrease of cytosolic Ca2+ concentration. 4. Oscillatory changes in cell membrane conductance and capacitance were consistently observed even without applying secretagogues when the cells were dialysed with a solution containing GTP gamma S (50-100 microM) and cytosolic free Ca2+ ions weakly buffered at about 10(-6) M with a low dose of EGTA and CaCl2. 5. The peak amplitude of changes in cell membrane capacitance induced by ACh or CCK-8, with or without GTP gamma S in the cytosol, varied between 200 and 1000 fF, thereby suggesting that 20-100 zymogen granules can fuse with the luminal cell membrane in response to these agonists in rat exocrine pancreatic acinar cells.

Regulation of P2y-purinoceptor-mediated prostacyclin release from human endothelial cells by cytoplasmic calcium concentration

1. ATP and ATP analogues induced prostacyclin (PGI2) secretion from human cultured umbilical vein endothelial cells. 2. The threshold active concentration for ATP was less than or equal to 1 microM. The rank order of potency of analogues was 2-chloroadenosine 5'-triphosphate (2-ClATP) greater than 2-methylthioadenosine 5'-triphosphate (2-MeSATP) greater than ATP greater than ADP, while adenosine 5'-(alpha,beta-methylene)triphosphonate, AMP and adenosine were inactive, indicating the presence of P2y-purinoceptors. 3. In contrast to their actions on P2y-receptors in guinea-pig taenia coli, isopolar analogues of 2-methylthioadenosine 5'-(beta, gamma-methylene)triphosphonate were less effective than ATP. 4. ATP and ATP analogues increased intracellular free calcium ions, [Ca2+]i, giving a rapid transient peak due predominantly to release from intracellular stores, followed by a maintained steady-state elevated level due to influx. 5. The dose-response curves for peak [Ca2+]i induced by ATP, 2-ClATP and 2-MeSATP were very similar to those for PGI2 production. 6. Elevations of [Ca2+]i, above a threshold value of 0.8-1 microM, were necessary for PGI2 production in response to P2y-receptor activation. 7. The dose relationships between PGI2 release and peak [Ca2+]i were equivalent whether [Ca2+]i was raised by ionomycin or via P2y-receptor activation by ATP or 2-ClATP, indicating that elevations of [Ca2+]i provide the major, if not the exclusive intracellular pathway for P2y-purinoceptor-mediated PGI2 synthesis.

Heterogeneous calcium and adenosine triphosphate distribution in calcium-permeabilized human red cells

1. Calcium permeabilization of inosine-fed human red cells using the divalent cation ionophore A23187 induces pump-leak steady states in which the mean total calcium content of the cells may be held below electrochemical equilibrium for hours. A new method developed to detect and separate cells with different calcium contents revealed a striking heterogeneity of calcium contents in subpopulations of cells in pump-leak steady state (García-Sancho & Lew, 1988a). Most of the mean total cell calcium was found within a fraction of cells rendered dense by the separation procedure (H cells), with relatively little within the remaining light cells (L cells). The experiments in this paper were designed to study the nature and origin of the observed heterogeneity. 2. The fraction of steady-state H cells increased, and the mean ATP content of the cells fell, both linearly, as calcium influx was increased. The H/L divide is therefore the result of a continuous variation in cell properties. When calcium influx was above about 30 mmol/(l cells.h), all cells became dense, calcium distribution was at or near equilibrium, and cell ATP was 0.1-0.2 mmol/l cells. 3. Inosine-fed cells, subjected to ionophore-mediated net calcium influx of 13-15 mmol/(l cells.h), attained a steady state with mean calcium contents far below equilibrium. After ionophore removal and reincubation in calcium-free media, the initial calcium efflux was only a fraction of that required to sustain the previous steady state (less than 25% for H cells, and less than 2% for L cells). The ATP content of L cells was normal whereas that of H cells was irreversibly reduced. These results revealed a paradoxical discrepancy between leak influx and calcium pump efflux in H and L cells which were supposed to have been in steady-state pump-leak balance. 4. The changes in cell calcium and ATP were followed in time after calcium permeabilization to characterize the development of steady-state heterogeneity. Calcium influx triggered a sharp peak in the H cell fraction within 15 s of permeabilization. The mean calcium content of H cells increased towards steady-state values as their fraction decreased; most other cells transferred from H to L density fractions (HL cells) within the first 5 min of permeabilization. 5. In substrate-starved cells calcium influx triggered an immediate fall in cell ATP, steeper in H cells than in L cells. The initial calcium and density transients were unattected.(ABSTRACT TRUNCATED AT 400 WORDS)

Botulinum toxin-induced ADP-ribosylation and inhibition of exocytosis are unrelated events

The hypothesis that inhibition of secretion by botulinum neurotoxin type D occurs by an intracellular process involving ADP-ribosylation has been directly tested by measuring both the extent of inhibition of secretion and of ADP-ribosylation in the same cells. Although the inhibitory effect of unpurified toxin closely parallels intracellular ribosylation, the two events are clearly unrelated, as using purified D and C3 toxins together with their antibodies, each of these events can be either stimulated or inhibited independently of each other.

Control of exocytosis from adrenal chromaffin cells

1. Calcium-dependent exocytosis of catecholamines from intact and digitonin-permeabilized bovine adrenal chromaffin cells was investigated. 2. 45Ca2+ uptake and secretion induced by nicotinic stimulation or depolarization in intact cells were closely correlated. The results provide strong support for Ca2+ entry being the trigger for exocytosis. 3. Experiments in which the H+ electrochemical gradient across the intracellular secretory granule (chromaffin granule) membrane was altered indicated that the gradient does not play an important role in exocytosis. 4. Ca2+ entry into the cells is associated with activation of phospholiphase C and a rapid translocation of protein kinase C to membranes. 5. The plasma membrane of chromaffin cells was rendered permeable to Ca2+, ATP, and proteins by the detergent digitonin without disruption of the intracellular secretory granules. In this system in which the intracellular milieu can be controlled, micromolar Ca2+ directly stimulated catecholamine secretion. 6. Treatment of the cells with phorbol esters and diglyceride, which activate protein kinase C, enhanced phosphorylation and subsequent Ca2+-dependent secretion in digitonin-treated cells. 7. Phorbol ester-induced secretion could be specifically inhibited by trypsin. The experiments indicate that protein kinase C modulates but is not necessary for Ca2+-dependent secretion.

The action of volatile anaesthetics on stimulus-secretion coupling in bovine adrenal chromaffin cells

1. The action of four volatile anaesthetics, ethrane, halothane, isoflurane and methoxyflurane on stimulus-secretion coupling has been studied in isolated bovine adrenal medullary cells. All four agents inhibited the secretion of adrenaline and noradrenaline evoked by 500 microM carbachol at concentrations within the anaesthetic range. Total catecholamine secretion induced by stimulation with 77 mM potassium was also inhibited but at higher concentrations. All four agents inhibited the 45Ca influx evoked by stimulation with 500 microM carbachol and the 45Ca influx in response to K+-depolarization. 2. When total catecholamine secretion in response to potassium or carbachol was modulated by varying extracellular calcium or by adding halothane or methoxyflurane to the incubation medium, the amount of catecholamine secretion for a given Ca2+ entry was the same. 3. The action of methoxyflurane on the relationship between intracellular free Ca and exocytosis was examined using electropermeabilised cells, which were suspended in solutions containing a range of concentrations of ionised calcium between 10(-8) and 10(-4)M. The anaesthetic had no effect on the activation of exocytosis by intracellular free calcium. 4. Halothane and methoxyflurane inhibited the carbachol-induced secretion of catecholamines in a non-competitive manner. 5. Halothane and methoxyflurane inhibited the increase in 22Na influx evoked by carbachol. For halothane and methoxyflurane this inhibition of Na influx appears to be sufficient to account for the inhibition of the evoked catecholamine secretion. 6. We conclude that the volatile anaesthetics ethrane, halothane, isoflurane and methoxyflurane inhibit the secretion of adrenaline and noradrenaline induced by carbachol at concentrations that lie within the range encountered during general anaesthesia. In addition all four also inhibit the secretion of catecholamines induced by depolarization with 77 mM K+ but at much higher concentrations. The decrease in Ca influx caused by methoxyflurane accounts fully for the decrease in secretion in response to depolarization with potassium. Similar actions at synapses within the CNS may underlie the general anaesthetic effects of these agents.

Effects of platelet activating factor on calcium-lipid interactions and lateral phase separations in phospholipid vesicles

Recent studies localizing the inflammatory mediator, platelet activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine), to the membranes of stimulated neutrophils, raise the possibility that PAF may, in addition to its activities as a mediator, alter the physical properties of membranes. This, and the increasing evidence that calcium-lipid interactions may have central importance in membrane organizational structure and in functions of cell homeostasis and stimulus-response coupling, prompted us to study the effects of PAF on calcium-lipid interactions in lipid vesicles. Using fluorescence polarization of dansylated probes located in the glycerol portion of the membrane bilayer, PAF (at a concentration as low as 1 mol%) was shown to reduce membrane rigidification significantly during calcium-induced lateral phase separations. This effect of PAF was structurally dependent on both the 1-position alkyl linkage and the 2-position acetyl group as shown by studies of related lipid analogs. Furthermore, using a self-quenching probe, it was shown that inhibition of lateral phase separation did not account for this reduction in the calcium-induced membrane rigidification attributed to PAF. Data suggest that PAF at low concentrations may alter phospholipid head packing and, thereby, change membrane surface features during calcium-lipid interactions, effects which may ultimately explain some of its biological actions.

Vesicle-membrane fusion. Observation of simultaneous membrane incorporation and content release

Vesicle fusion, the central process of neurotransmitter release and hormonal secretion, is a complex process culminating in simultaneous incorporation of vesicle membrane into the plasma membrane and release of the vesicular contents extracellularly. This report describes simultaneous observation of membrane incorporation and content release using a model system composed of a planar bilayer and dye-filled vesicles.

Evidence implicating protein kinase C in exocytosis from electropermeabilized bovine chromaffin cells

The calcium sensitivity of exocytosis from electro-permeabilized chromaffin cells is increased by activators of protein kinase C, such as TPA and certain phorbol esters, diacylglycerols, and mezerein. A range of putative inhibitors of protein kinase C block both the phorbol ester-sensitive component of secretion and also the underlying insensitive component. These inhibitors are also shown to inhibit medulla protein kinase C activity in vitro. The extent of secretion is reduced when electro-permeabilized cells are exposed to Ca2+ levels much in excess of 50 microM. The onset of inhibition is faster than the relatively slow rate of Ca-dependent exocytosis and is insensitive to inhibitors of proteolysis. Adrenal medulla protein kinase C activity is also irreversibly inhibited by high Ca2+ concentrations. Both the secretory response and the protein kinase C activity in vitro have similar nucleotide and cation specificities. Although these data do not definitely establish an involvement of protein kinase C in exocytosis, none argue against it.

Developmentally regulated compartmentalization of adenylate cyclase in Dictyostelium discoideum

Adenylate cyclase of aggregation phase Dictyostelium discoideum is activated by extracellular adenosine 3', 5'-cyclic monophosphate (cAMP), and the cAMP synthesized is secreted. The distribution of the enzyme was determined in sucrose gradients loaded with whole cell lysates. Cell lysates prepared after 4.5 hr of starvation revealed membranes containing adenylate cyclase at 44% and 33% sucrose. The activity of the latter peak was detected in the presence of the detergent (CHAPS), 3-(3-cholamidopropyl) dimethylammonio-3-propanesulfonate, which inhibited the activity of the former to some extent. Adenylate cyclase activity of the 2 peaks differed with respect to solubility in CHAPS and their kinetics. The 44% sucrose region of the gradient contained the bulk of the plasma membranes, as judged by a cell surface glycoprotein marker (contact site A). The 33% peak is composed of small vesicular structures, as determined by electron microscopy. The distribution of adenylate cyclase activity detected in sucrose gradients shifted from the 33% to the 44% sucrose peak during development. In addition, the 44% peak became increasingly resistant to the inhibitory effect of CHAPS. Both changes were accelerated by extracellular cAMP, but only the latter was abolished when the production of endogenous cAMP was inhibited by caffeine. Pulsing cells with cAMP overcame the inhibitory effect of caffeine.

Calcium homeostasis in digitonin-permeabilized bovine chromaffin cells

The regulation of cytosolic calcium was studied in digitonin-permeabilized chromaffin cells. Accumulation of 45Ca2+ by permeabilized cells was measured at various Ca2+ concentrations in the incubation solutions. In the absence of ATP, there was a small (10-15% of total uptake) but significant increase in accumulation of Ca2+ into both the vesicular and nonvesicular pools. In the presence of ATP, the permeabilized cells accumulated Ca2+ into carbonyl cyanide m-chlorophenyl hydrazone (CCCP)-sensitive and -insensitive pools. The CCCP-sensitive pool--mainly mitochondria--was active when the calcium concentration was greater than 1 microM and was not saturated at 25 microM. The Ca2+ sequestered by the CCCP-insensitive pool could be inhibited by vanadate and released by inositol trisphosphate, a combination suggesting that this pool was the endoplasmic reticulum. The CCCP-insensitive pool had a high affinity for calcium, with an EC50 of approximately 1 microM. When the Ca2+ concentration was adjusted to the level in the cytoplasm of resting cells (0.1 microM), the presumed endoplasmic reticulum pool was responsible for approximately 90% of the ATP-stimulated calcium uptake. At a calcium level similar to the acetylcholine-stimulated level in intact cells (5-10 microM), most of the Ca2+ (greater than 95%) went into the CCCP-sensitive pool.

Adenosine 3',5'-cyclic monophosphate-mediated enhancement of calcium-evoked prolactin release from electrically permeabilised 7315c tumour cells

1. The 7315c tumour cell was used as a model system for the investigation of adenosine 3',5'-cyclic monophosphate (cyclic AMP)-mediated enhancement of calcium-evoked prolactin release. 2. 7315c cells were permeabilised by subjecting the cells to intense electric fields. Studies investigating the penetration of the dye ethidium bromide indicated that the cells were completely permeabilised after 2 discharges of 2000 volts and that the pores remained open for at least 30 min before beginning to reseal. These permeabilisation parameters were consistent with those which gave maximal calcium-stimulated prolactin release. 3. In the absence of calcium and in the presence of EGTA (1 mM), permeabilised 7315c cells secreted prolactin at a rate of 0.23 ng min-1 per 10(6) cells. When EGTA was replaced by 1.5 mM calcium, permeabilised cells secreted prolactin at a rate of 2.20 +/- 0.30 ng min-1 per 10(6) cells in the first 5 min of exposure. Maximal calcium-dependent prolactin secretion from permeabilised cells occurred at 37 degrees C. 4. The amount of prolactin secreted, in a 5 min incubation at 37 degrees C, from permeabilised cells depended upon the free calcium concentration in the permeabilisation medium. Calcium stimulated prolactin release from permeabilised cells in the concentration range 0.1-10 microM (half maximal = 5.8 microM). When permeabilised cells were exposed to cyclic AMP (100 microM) for 5 min prior to and during a 5 min challenge with various concentrations of calcium, the amount of prolactin secreted at each effective concentration of calcium was increased. However, cyclic AMP did not alter the potency of calcium as a stimulant of prolactin secretion. 5. The results suggest that cyclic AMP potentiates calcium-evoked secretion from 7315c cells, not by increasing the entry of calcium into the cytosol, but at a step in the secretory process, distal to calcium entry, which modulates the ability of an increase in cytosolic calcium concentration to stimulate prolactin release.

Calcium and neuronal function

Calcium is unique among metals because its ions have a very large concentration gradient across the plasma membrane of all cells, from 10(-3) M Ca2+ outside, to 10(-7) M Ca2+ inside. This gradient is maintained by the use of metabolic energy through ion pumping, and its existence allows cells to use transient increases in the intracellular Ca2+ concentration as signals, which regulate cell function. In neurones these Ca signals are initiated by electrical activity (action potentials) which open voltage-dependent Ca channels in the plasma membrane, allowing Ca to enter the cell. Intracellular Ca signals can also be produced by transmitters at synapses, which open Ca channels, either directly, or indirectly by causing local depolarization and the opening of voltage-dependent Ca channels. The main effects of Ca signals on neurones are to alter their electrical activity, by modifying the opening and closing of Na and K channels, and to stimulate the release of transmitter substance. Ca has a host of other effects, such as the regulation of metabolic activity, the regulation of cell growth, and the long-term modification of synaptic efficiency, and it is even implicated in the destruction of neurones.

The role of cytoplasmic pH in the inhibitory action of high osmolarity on secretion from bovine adrenal chromaffin cells

Elevated osmolarity is known to inhibit secretion from a wide range of cells including bovine adrenal chromaffin cells. The mechanism of this inhibition is unclear but the elevated osmolarity has been proposed to oppose an osmotic driving force involved in exocytotic fusion. Using the fluorescent indicators quene 1 and fura2, we monitored the effect of elevated osmolarity on cytoplasmic pH (pHi) and cytoplasmic free Ca2+ [( Ca2+]i). Elevated osmolarity increased both pHi and [Ca2+]i, but had no effect on the [Ca2+]i rise elicited by either K+ or nicotine. Elevating pHi with NH4Cl was shown to inhibit secretion from chromaffin cells. The elevation of pHi by hyperosmolar solutions is proposed as one of the mechanisms by which elevated osmolarity inhibits secretion.

Osmotic forces are not critical for Ca2+-induced secretion from permeabilized human neutrophils

In order to examine the role of osmotic forces in degranulation, the effects of solutes and osmolality on granule secretion were explored using both FMLP-stimulated, intact neutrophils and Ca2+-stimulated, permeabilized cells. We employed a HEPES-based buffer system which was supplemented with: a) permeant (KCl or NaCl) or impermeant (Na-isethionate or choline-Cl) ions, or b) permeant (urea) or impermeant (sucrose) uncharged solutes. Intact and permeabilized cells had significantly different solute requirements for degranulation. FMLP-stimulated release from intact cells was supported by NaCl or Na-isethionate greater than KCl greater than choline-Cl or sucrose greater than urea. In contrast, the rank order of Ca2+-stimulated release from permeabilized cells was choline-Cl greater than Na-isethionate, KCl, or NaCl greater than sucrose greater than urea. Hypo-osmotic conditions caused increased levels of background granule release from both intact and permeabilized neutrophils. However, hypo-osmolality inhibited both FMLP-stimulated degranulation from intact cells and Ca2+-induced release from permeabilized neutrophils. While hyperosmotic conditions inhibited stimulated release from intact cells, this inhibition was much less pronounced in permeabilized cells when the granules were directly exposed to these solutions. In fact, hyperosmotic sucrose greatly enhanced Ca2+-induced secretion. Although isolated specific and azurophil granules showed some lytic tendencies in hypo-osmotic buffers, the overall stability of the isolated granules did not indicate that swelling alone could effect degranulation. These results suggest that degranulation in permeabilized cells is neither due to nor driven by simple osmotic forces (under resting or stimulated conditions) and emphasize differences obtained by bathing both the granules and plasma membrane (as opposed to membranes alone) in various solutes.

Calmodulin- and protein phosphorylation-independent release of catecholamines from PC-12 cells

Catecholamine secretion from PC-12 cells can be triggered by agents that increase intracellular Ca2+ and is enhanced by phorbol esters and agents that elevate intracellular cAMP concentrations. In mutant PC-12 cells lacking cAMP-dependent protein kinase (PK-A) in which protein kinase C (PK-C) was down-regulated, Ca2+-dependent secretion occurred normally but was no longer enhanced by cAMP or phorbol esters. In digitonin-permeabilized PC-12 cells that lacked PK-C and PK-A, a range of calmodulin (CaM) inhibitors failed to block Ca2+-triggered catecholamine release. Moreover, Mn2+, a CaM activator, failed to trigger catecholamine release whereas Ba2+, which does not activate CaM, supported secretion. These results indicate that the basic mechanism of stimulus/secretion coupling in PC-12 cells does not absolutely require a regulated protein phosphorylation- or calmodulin-dependent step.

Introduction of definite amounts of nonpermeant molecules into living cells after electropermeabilization: direct access to the cytosol

The possibility of introducing definite amounts of nonpermeant molecules into electropermeabilized living cells has been approached by quantifying the amounts of Lucifer Yellow (LY; a 457-Da highly fluorescent molecule) and Phytolacca americana (Pokeweed) antiviral protein (PAP; a 30,000-Da ribosome-inactivating protein) retained by the cells after closure of the electric-field-induced transient structures of permeation. Without the electropermeabilization, these two molecules enter the cell only in very small amounts by fluid-phase pinocytosis. Under our experimental conditions, using the NIH 3T3 cells, the intracellular LY concentration can reach a value equivalent to the extracellular concentration and can be regulated by controlling the external concentration. We describe the use of LY in a rapid and efficient test for the determination of the best electrical-shock conditions of other cell lines. After electropermeabilization, PAP is 2 X 10(5) times more cytotoxic. Its toxicity can be detected at external concentrations (10(-11) M) corresponding to less than 10 internalized molecules per electropermeabilized cell. Therefore, after electropermeabilization, the nonpermeant molecules have a direct access to the cytosol and the biological effect of nonpermeant substances can be revealed.

Aggregation of IgE receptors induces degranulation in rat basophilic leukemia cells permeabilized with alpha-toxin from Staphylococcus aureus

A method has been developed by which rat basophilic leukemia (RBL) cells can be permeabilized to small molecules while maintaining their ability to degranulate in response to aggregation of IgE receptors. alpha-Toxin was isolated from culture supernatants of Staphylococcus aureus by precipitation with (NH4)2SO4 and chromatography on phenyl-Sepharose. The isolated toxin binds to the plasma membrane of RBL cells and polymerizes to form a transmembrane pore that allows small molecules (Mr less than 1000), but not macromolecules, to diffuse freely across the membrane. There was no spontaneous release of the contents of RBL cell secretory granules during permeabilization or subsequent incubations. Substantial IgE receptor-mediated exocytosis occurred in the absence of Ca2+, and degranulation was maximal at 0.1-1.0 microM Ca2+, the physiologically important range of Ca2+ concentrations. Using these permeabilized cells, small molecules (i.e., substrates and inhibitors of various enzymes) normally excluded by the plasma membrane can be introduced into the cell. Moreover, the intracellular concentration of ions (such as Ca2+) can be precisely controlled. This method will allow a detailed examination of the individual biochemical events involved in degranulation of mast cells.

Molecular mechanisms of exocytosis: the adrenal chromaffin cell as a model system

1. The release of neurotransmitters, hormones, and enzymes involves exquisitely regulated events which ultimately result in the fusion of the secretory vesicle with the cell's plasma membrane, releasing the vesicle contents into the extracellular space. 2. The biochemical and cellular mechanisms mediating exocytosis have been extensively studied in a model system of primary cultured adrenal chromaffin cells. 3. This paper briefly reviews current understanding, and directions of future studies in exocytosis using this model system.

Dual regulation of ACTH secretion by guanine nucleotides in permeabilized AtT-20 cells

1. We have examined the effects of guanine nucleotides on ACTH secretion from digitonin-permeabilized AtT-20 cells, with the aim of analyzing the involvement of GTP-binding proteins (G proteins) in the secretory process. 2. AtT-20 cells permeabilized with 20 microM digitonin displayed calcium-dependent secretion. The EC50 of calcium was approximately 2 microM and the maximal stimulation was 350% of basal release. 3. Nonhydrolyzable guanine nucleotides also stimulated ACTH release, in a virtually Ca2+-free medium. The EC50 of guanosine 5'-(3-O-thio)triphosphate (GTP gamma S) was approximately 15 microM and the maximal stimulation was approximately 230% of basal release. The effects of calcium and guanine nucleotides were not additive. 4. In the presence of the inhibitory hormone, somatostatin guanine nucleotides inhibited the calcium-stimulated secretion. 5. Both the stimulatory and the inhibitory effects on secretion of guanine nucleotides were independent of changes in cyclic AMP (cAMP) and calcium. It is suggested that G proteins influence an unknown step in the secretion process, which would be near or at the exocytotic site. 6. The results can be explained by assuming the existence of two types of G proteins, one with stimulatory effects on exocytotic release (GeS) and another with inhibitory effects (GeI).

Ionic and permeability requirements for exocytosis in vitro in sea urchin eggs

We study exocytosis in the planar isolated cortex of the egg of the sea urchin Lytechinus pictus. Solutions bathing the exocytotic apparatus need not contain appreciable amounts of ions: fusion follows addition of submicromolar calcium to solutions containing only nonelectrolyte. We examine the effects of altering the granule membrane permeability to small molecules with ionophores and digitonin. Introducing holes in the secretory granule membrane to the extent of allowing free passage of small molecules does not cause secretion in vitro. We add the amphipathic compound digitonin at 12 to 15 microM concentrations and demonstrate that the granule membrane can become permeable to lucifer yellow, yet that granules remain intact. Granules still undergo exocytosis after digitonin treatment at such concentrations upon subsequent addition of calcium. Higher concentrations of digitonin lead to granule content swelling and vesicle bursting. We conclude that cortical granule hydration during exocytosis is not mediated by small ionic channels.

Substrates for adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase in the rat pituitary gland

1. Substrates for cAMP-dependent protein kinase were investigated in anterior, intermediate, and neural lobes of the rat pituitary gland. In a cell-free assay system, cAMP increased phosphorylation of 17 K, 33 K, and 60 K macromolecules of the anterior lobe, 17 K, 33 K, 60 K, and 80 K macromolecules of the intermediate lobe, and 60 K, 80 K, and 85 K macromolecules of the neural lobe. 2. Other nucleotides were tested in the intermediate lobe; 8 Br-cAMP mimicked cAMP, cGMP was much less effective than cAMP or 8 Br-cAMP, and 5'-AMP showed no significant effect. The purified catalytic subunit of cAMP-dependent protein kinase evoked the same phosphorylation pattern as the endogenous kinase. 3. Maximum cAMP-dependent phosphorylation occurred at between 1 and 2 min of incubation; after 20 min, phosphorylation was reduced by 80%. This suggests the presence of phosphatase activity in the intermediate lobe. 4. When tested upon dispersed intermediate lobe cells permeabilized by high-voltage electrical discharges, cAMP increased phosphorylation of the 17 K and 33 K macromolecules.

Effects of trypsin on secretion stimulated by micromolar Ca2+ and phorbol ester in digitonin-permeabilized adrenal chromaffin cells

1. Catecholamine secretion from digitonin-treated chromaffin cells is stimulated directly by micromolar Ca2+ in the medium. The permeabilized cells are leaky to proteins. 2. In this study trypsin (30-50 micrograms/ml) added to cells after digitonin treatment completely inhibited subsequent Ca2+-dependent catecholamine secretion. The same concentrations of trypsin did not inhibit secretion from permeabilized cells if trypsin was present only prior to cell permeabilization. 3. The data indicate that trypsin entered digitonin-treated chromaffin cells which were capable of undergoing secretion and that an intracellular, trypsin-sensitive protein is involved in secretion. Chymotrypsin was less potent but had effects similar to those of trypsin. 4. The enhancement of Ca2+-dependent secretion from permeabilized chromaffin cells induced by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) was inhibited by trypsin added simultaneously with Ca2+ to permeabilized cells at concentrations (3-10 micrograms/ml) which had little or no effect on Ca2+-dependent secretion from cells untreated with TPA. Ca2+-dependent secretion in TPA-treated cells was reduced by trypsin only to the level that would have occurred in cells not treated with TPA. Trypsin reduced the large TPA-induced increment of membrane-bound protein kinase C.

Enzyme stimulation upon fertilization is revealed in electrically permeabilized sea urchin eggs

Sea urchin eggs and embryos subjected to high-voltage electric discharge in a medium mimicking the intracellular milieu retain their structural integrity and remain permeable, permitting substrates to enter the cytoplasm and thus assay of enzyme activity. At saturating concentrations of substrates, five of six enzymes assayed for more active (three to fifteen times) in permeabilized embryos than in permeabilized eggs, but no fertilization-related differences are seen in homogenates prepared from these same permeabilized cells. Furthermore, enzyme activity in homogenates always exceeds that in the permeabilized cell suspensions. This difference in enzyme reaction rates between unfertilized eggs and fertilized eggs is not due to differences in the diffusibility of substrates into the permeabilized cells. The activity of glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate:NADP+ 1-oxidoreductase, EC 1.1.1.49) in permeabilized cells was studied in greater detail and has the following characteristics. (i) Regulation of activity persists during early development. (ii) This regulation is not mediated by diffusible allosteric agents. (iii) Stimulation at fertilization is initiated by a rise in intracellular calcium and is further promoted by cytoplasmic alkalinization. (iv) The microenvironment experienced by this enzyme intracellularly differs from that of the enzyme in homogenates as evidenced by markedly different pH vs. activity profiles. These results indicate that the regulatory status of enzymes is preserved in electrically permeabilized cells and suggest that this regulation depends on some cell structural feature(s) that is (are) destroyed upon homogenization.

Requirement of ATP for exocytotic release of catecholamines from digitonin-permeabilized adrenal chromaffin cells

Cultured chromaffin cells were preincubated with digitonin to deplete endogenous ATP from the cell cytoplasm. Catecholamine release from these digitonin-pretreated cells was then studied in the presence and absence of exogenous ATP to elucidate a possible involvement of the cytoplasmic ATP in the exocytotic process. The preincubation of the cells with digitonin in the ATP-free permeabilizing medium resulted in a marked decline of the releasing response to a calcium challenge. Furthermore, the declined activity of catecholamine release caused by digitonin pretreatment was restored by the presence of ATP, but not by other adenine nucleotides, and this recovery was observed in a manner dependent on the concentration of ATP. These findings, therefore, seem to indicate that a decrease in the releasing activity of the digitonin-pretreated cells may be due to the removal of endogenous ATP from the cytoplasmic space of the cells, thus suggesting that the cytoplasmic ATP may be involved in the exocytotic mechanism of catecholamine secretion.

Aggregation of chromaffin granules by calpactin at micromolar levels of calcium

Several cytosolic proteins bind to secretory granule membranes in a Ca2+-dependent manner and thus may be involved in the mediation of membrane interactions during exocytosis. One of these proteins, calpactin, is a tetramer consisting of two heavy chains of relative molecular mass (Mr) 36K (p36) and two light chains of 10K (p10). We report here that calpactin promotes the Ca2+-dependent aggregation and fatty acid-dependent fusion of chromaffin granule membranes at a level of Ca2+ that is lower than that reported for other granule-aggregating proteins, and which parallels the Ca2+ requirement for secretion from permeabilized chromaffin cells. We found subunits of calpactin to be inactive in promoting granule aggregation. Two distinct 33K proteolytic fragments of p36, differing at their N termini, also promote granule aggregation but with different Ca2+ sensitivities from calpactin. These differences suggest that the N-terminal portion of p36 modulates the Ca2+/lipid binding sites in the core portion of p36 (ref.5).

The use of cell permeabilization for studying the mechanisms of action of angiotensin II in the adrenal cortex and vascular smooth muscle

Some applications of the technique of electropermeabilization to the study of the mechanisms of action of angiotensin II in two of its physiological target cells, the adrenal zona glomerulosa and the vascular smooth muscle cell, are described. The technique proved useful for characterizing the second messenger role of inositol 1,4,5-trisphosphate, its metabolic pathways and the effect of Ca2+ on steroidogenesis.